The present invention relates to resource allocation in multi-cellular networks.
The exponentially increasing demand for data (particularly in the downlink) has ensured that future wireless cellular networks will be interference-limited. This necessitates a careful handling of inter-cell interference. Indeed substantial performance gains are possible if inter-cell interference is managed via coordinated resource allocation across multiple cells. Studies on coordinated processing assumed that both data and channel state information of all users are shared in real-time. However, in practice coordination is possible only on a per-cluster basis. Furthermore, the limited backhaul bandwidth essentially prevents real-time data sharing. Thus, it is reasonable to assume that each user can be served by only one base station. Nevertheless, downlink beam-vectors can still be optimized based on the inter-cell channel qualities and has been considered.
The application of linear transmit precoding over interference limited networks has been bolstered by recent degree-of-freedom (DoF) optimality results for interference alignment schemes (that involve linear precoding) for time-varying or frequency-selective interference channels. More importantly, realizing its benefits, fourth generation cellular standards such as LTE-A—CoMP: Coordinated Multi-Point TX/RX and IEEE 802.16m—Multi-BS MIMO have enabled coordinated linear transmit precoding among multiple cells albeit based on limited exchange of channel state information (CSI). Unfortunately, the optimal multi-cell linear precoding design problems are known to be hard even when perfect and global CSI is available. In particular, the weighted sum rate optimization problem even in the SISO interference channel with perfect CSI was shown to be NP hard. Consequently distributed and iterative algorithms that seek sub-optimal solutions have been proposed under both perfect CSI and imperfect CSI.
Practical resource allocation problems are inherently mixed optimization problems. This is because while the precoders can often be matrices with arbitrary complex-valued entries (subject to power constraints), most of the resources that have to be assigned to the users (such as frequency sub-carriers, modulations, among others) are discrete in nature. In order to handle the discrete aspect of our resource allocation problems we leverage sub-modular optimization techniques that can often provide a worst-case guarantee even when a low-complexity greedy algorithm is employed. We consider a cluster of cells communicating over multiple orthogonal slots in a coordinated fashion such that each user is associated with (and is served by) a particular cell and where intra-cell interference is avoided by ensuring users associated with the same cell are not simultaneously served on the same slot. In the following, we will use the terms user and mobile device interchangeably. Similarly the terms base station and source are also used interchangeably. Under this setup, we formulate and analyze three important versions of practical downlink multi-cell coordinated resource allocation under the following practical constraints that significantly reduce the signaling overhead and will be ubiquitous in the emerging 4G cellular networks.
The first constraint is that in any scheduling interval each scheduled user can be served using only one or at-most two distinct modulations. In practical systems, each modulation must be one out of 4, 16 or 64 QAM. On other hand powerful Turbo codes of several distinct coding rates are available. Hence it is reasonable to assume that for each QAM alphabet, ideal outer codes of a continuum of coding rates in are available. In addition to the above constraint, each scheduled user can be served using only one rank, i.e., the ranks of all the precoding matrices used to serve a particular user in a scheduling interval must be identical. Thus, while a different precoding matrix can be used to serve a particular user on each of its assigned slots, all these matrices must have a common rank. In certain systems with more stringent signaling overhead constraints, in any scheduling interval each scheduled user can be served by only one distinct precoding matrix drawn from a pre-defined finite codebook, over all slots assigned to that user.